Electronic power switches are widely used in the power electronics field, such as IGBT bridges in a chopper, IGBT bridges and high and intermediate voltage frequency converters, for their excellent electric control property. Electronic power switches generally have lower voltage withstanding. If several power switches are connected in series in a high voltage circuit, the later switched power switches during switched on or the former switched power switches during switched off will be subjected to too high voltage due to different switching time of different switches and overvoltage induced in loop inductors by di/dt, and thereby be damaged, which makes applying power switches in series quite difficult in development. Therefore, one of the common methods presently employed in high voltage circuits is by frequency-shift master transformer, scheme of IGBT A-D-A bridge in series, i.e. “H” bridge superposition scheme. (refer to “Electric Driving”, published by Tianjin Research Institute of Electric Driving, 2000, 2, p. 3–6). These schemes have the disadvantages of complex construction, difficult manufacturing, high cost, and high error rates.